Systems that rely upon sensing optical energy to discern information are known in the art and have many applications. Exemplary applications might include an optical-based system to determine range between the system and a target object, or to identify and recognize features of a target object. Many such systems acquire two-dimensional or intensity-based information, and rely upon an intensity image of light reflected from a target object. Such luminosity-based systems can use ambient light falling upon the target object, or may actively generate light that is directed toward the target object.
Unfortunately, it is difficult to accurately determine distance solely from the amplitude and brightness of an intensity image. For example, in a range finding system, a highly reflecting target object that is father away from the system can produce a greater amplitude signal than a nearer target object that is less reflective. The result would be that the more distant, shiny, object is erroneously reported as being closer to the system than the closer, duller, object. In a range finding system used to control robot machinery in an industrial setting, such errors may be intolerable for reasons of safety to nearby human operators. If such a system is used to identify and recognize different target objects, an object might be misidentified. Simply stated, two-dimensional intensity-based systems are very prone to measurement error.
The present invention may be used in a wide range of applications including, but not limited to, range finding systems. It may be useful at this juncture to briefly review such systems.
U.S. Pat. No. 6,323,942 (“the '942 patent”) to Bamji et al. entitled CMOS-COMPATIBLE THREE-DIMENSIONAL IMAGE SENSOR IC describes a three-dimensional range finding system that can determine range distance without reliance upon luminosity-based data, the entire content of which patent is incorporated herein by this reference. As disclosed in the '942 patent, such a system generates a depth map that contains the distance Z from each pixel in a CMOS-compatible sensor array to a corresponding location on a target object. Applicants refer to and incorporate by reference the '942 patent as background material. U.S. Pat. No. 6,580,496 (“the '496 patent”) to Bamji et al. entitled SYSTEMS FOR CMOS-COMPATIBLE THREE-DIMENSIONAL IMAGE SENSING USING QUANTUM EFFICIENCY MODULATION describes the use of quantum modulation techniques and differential detectors suitable for a three-dimensional range finding system, the entire content of which patent is incorporated herein by this reference. In the '496 patent the quantum efficiency of the substrate upon which differential CMOS sensors were fabricated was modulated synchronously with optical energy emitted from an energy source. Applicants refer to and incorporate by reference the '496 patent as background material.
FIG. 1 is a block diagram of a three-dimensional range finding system 10 as exemplified by the '942 patent or the '496 patent. Such systems determine distance Z between the system and locations on target object 20. System 10 may be fabricated upon a single IC 30, requires no moving parts, and relatively few off-chip components, primarily a source of optical energy 40, e.g., a light emitting diode (LED) or laser source, and associated optics 50. If desired, laser source 40 might be bonded onto the common substrate upon which IC 30 is fabricated.
System 10 includes an array 60 of pixel detectors 70, each of which has dedicated circuitry 80 for processing detection charge output by the associated detector. As used herein, the terms “detector”, “photodiode detector”, “pixel” and “pixel detector” may be used interchangeably. Array 60 might include 100×100 pixels 70, and 100×100 associated detector processing circuits 80. One will appreciate that other configurations may be used. IC 30 preferably also includes a microprocessor or microcontroller unit 90, RAM and ROM memory, collectively 100, a high-speed distributable clock 110, and various computing and input/output (I/O) circuitry 120. System 10 preferably further includes a lens 130 to focus light reflected from target object 20 upon pixels 70 in array 60. As noted in the above-mentioned patents, controller unit 90 may carryout distance-to-object and object velocity calculations and can output such calculations as DATA, for use by a companion device, if desired. As seen in FIG. 1, substantially all of system 10 may be fabricated upon CMOS IC 30, which enables shorter signal paths, and reduced processing and delay times. Also shown in FIG. 1 is ambient light that is present in the environment in which system 10 and target object 20 are found. As described herein, high levels of ambient light relative to levels of light from energy source 40 can be detrimental to reliable operation of system 10.
In brief, microprocessor 90 can calculate the roundtrip time for optical energy from source 40 to travel to target object 20 and be reflected back to a pixel 70 within array 60. This time-of-flight (TOF) is given by the following relationship:Z=C·t/2  eq. (1)where C is velocity of light.
Thus, without reliance upon luminosity information, system 10 can calculate that Z1=C·t1/2, Z2=C·t2/2, Z2=C·t3/2, and so on. The correct Z distances are obtained, even if more distant regions of target object 20 happen to be more reflective than nearer regions of the target object.
The ability of system 10 to determine proper TOF distances Z can be impacted when the magnitude of ambient light is large relative to the magnitude of reflected light from source 40. What occurs is that the various pixels 70 respond to incoming optical energy that represents the real signal to be measured (e.g., active energy originating from source 40 and reflected by target object 20), and also respond to ambient light. The depth resolution of each pixel, i.e., the accuracy of the distance measurement, is determined by the system signal-to-noise ratio (SNR). Even if ambient light could be measured and subtracted from the total signal, its noise component (e.g., shot noise) would still degrade system performance. Further, the presence of ambient light can have even more severe consequences by causing the pixel detector to saturate.
In a so-called differential pixel detector, the active optical energy contributes to both a differential mode signal and a common mode signal, while ambient light only contributes to the common mode signal. While differential pixel detectors can exhibit higher SNR than single-ended pixel detectors, the presence of strong ambient light, sunlight perhaps, can degrade the performance of differential pixel detectors, as will now be described. As used herein, the term “differential detector” refers to a detector that responds to two input parameters. For example in the '496 patent, the differential detectors responded to amplitude of incoming optical energy and to phase of such energy relative to energy output by emitter 40; see FIG. 1 herein. Typically the singular term “pixel” will refer to a pair of differential photodetectors, for example first and second photodiode detectors DA and DB.
Incoming optical energy falling upon a pixel detector 70 generates an extremely small amount of photocurrent (or photocharge), typically on the order of picoamps (10−12 amps). Such detection current signals are too small in magnitude to be measured directly. It is known in the art to provide pixel detectors that operate in a direct integration mode by which optical energy induced photocurrent is integrated on a capacitor and the final capacitor charge or voltage is readout at the end of an integration interval. A capacitor Cx has finite maximum charge capacity Qmax defined by:Qmax=Cx·Vswing  eq.(2)where Cx is the total capacitance and Vswing is the maximum voltage swing across the capacitor. A pixel detector is said to be in saturation when the total charge integrated on the capacitor exceeds the maximum charge capacity, in which case no useful information can be readout from that pixel.
A differential pixel detector (e.g., detectors 70 in FIG. 1) may be represented as shown generically in FIG. 2A, in which modulation circuitry has been omitted for simplicity. Each pixel 70 has a differential structure with two perhaps identical reset and readout circuit components denoted A and B. Components A and B may be considered as part of the pixel 70 or as part of the pixel's associated circuitry 80. For ease of depictions, the photodetector pair comprising each differential pixel 70 is shown as photodiodes DA and DB, but other detector structures could be used instead, for example photogate structures. Capacitors CA and CB are shown in parallel with diodes DA and DB and represent detector parasitic capacitance and/or dedicated fixed value capacitors.
Referring briefly to FIG. 1, within system 10 microprocessor 90 causes optical energy source 40 to emit pulses of light that are directed by lens 50 toward target object 20. Some of this optical energy will be reflected back towards system 10 and will be focused by lens 130 onto pixels 70 within array 60. Incoming photon energy falling upon a detector 70 will cause photodetector pair DA and DB to generate a small amount of detection signal current that can be directly integrated by capacitors CA and CB. Before the start of integration, microprocessor 90, which may (but need not be) implemented on IC chip 30, will cause photodetectors DA and DB and their respective capacitors CA and CB to be reset to a reference voltage Vref. For the components shown in FIG. 2A, reset is caused by raising a reset signal Φreset (see FIG. 2B). During the integration time, photocurrent generated by detectors DA and DB respectively discharge associated capacitors CA, CB, as shown in FIG. 2B. During the integration time, the voltage seen at nodes SA, SB will decrease as a function of the photocurrent generated by the associated photodiode DA, DB. The magnitude of the photodiode-generated photocurrent will be a function of the amount of light energy received by the respective pixel 70 in array 60 in that the amount of light received by the pixel determines the final voltage on nodes SA and SB.
Readout circuitry is provided for circuit A and B, comprising transistors Tfollower and Tread. At the end of the integration time, which will be a function of the repetition rate of the optical pulses emitted from optical energy source 40, microprocessor 90 causes a readout signal Φread to go high. This enables the voltages on nodes SA and SB to be read-out of array 60, e.g., through a bitline. In the exemplary configuration of FIG. 2A, if the voltage on node SA or SB drops below a certain level denoted here as saturation voltage Vsat, the readout circuit cannot perform the reading operation properly. Therefore the dynamic range of such known differential pixel configuration shown in FIG. 2A is (Vref−Vsat), as depicted in FIG. 2B. While the waveforms in FIG. 2B depict a diminishing potential at nodes SA, SB as a function of photocurrent, one could instead configure the detector circuitry to charge rather than discharge a reference node potential.
But in addition to generating photocurrent in response to optical energy or active light (from emitter 40) reflected by target object 20, pixel 70 will also generate photocurrent in response to ambient light that is also integrated by capacitors CA, CB, thus affecting the potential at nodes SA, SB. FIG. 2B depicts two examples, showing the effect of relatively low magnitude ambient light, and relatively high magnitude of ambient light. In range finding applications, the difference (Afinal−Bfinal) generally contains range information, and common mode is of lesser importance. As shown in FIG. 2B, relatively weak ambient light does not cause the pixel to saturate, and at the end of integration time, the final voltages read-out from the pixel are above Vsat. But relatively strong ambient light discharges the associated capacitor potential rapidly, which saturates the pixel. Due to the saturation condition, the pixel does not output any useful result in that the differential voltage, which contained range information, is now zero. Thus, a very real problem with prior differential pixel detectors is that the dynamic range of the pixel is not sufficient to handle strong ambient light.
Thus, whereas CMOS sensors used in systems to acquire images generally rely upon strong levels of ambient light, CMOS sensors used in time-of-flight systems seek to reduce the effects of ambient light. As seen in FIG. 2B, the magnitude of ambient light can overwhelm detection of reflected optical energy, saturating the detectors. Image acquisition systems and time-of-flight systems that must function in environments exposed to strong ambient light or minimal ambient light may require a sensor dynamic range exceeding about 100 dB. In time-of-flight and similar applications in which ambient light is unnecessary, the detection effects of ambient light can be substantially reduced electronically. For example, the above-mentioned U.S. patent application Ser. No. 10/823,415 described various common mode reset techniques to substantially eliminate the effects of ambient light, as well as the effects of dark current. That application also disclosed techniques to mitigate the effects of low modulation contrast. As noted therein, common mode reset techniques reset common mode values to a fixed magnitude, while preserving differential mode detection signals on a per pixel photodiode detector basis.
Thus there is a need for a method and topology by which the dynamic range of a differential pixel detector can be enhanced such that degradation from ambient light is substantially reduced. Even in the presence of strong ambient light that might otherwise saturate the pixel, the differential response of the pixel should still be available. Preferably substantially all of the common mode signal should be removed, while retaining substantially all of the differential mode detection signals on a per pixel photodiode detector basis. Preferably such method and topology should introduce little or no KT/C capacitor noise, other than for parasitic capacitance effects. Finally, such method and topology should be implementable using CMOS such that the differential sensor array can still be fabricated on a common IC with associated range finding system.
Embodiments of the present invention provide such methods and circuit topologies.